Bird FossilsThe fossil record indicates that birds evolved from feathered ancestors within the theropod group of saurischian dinosaurs. True birds first appeared during the Cretaceous period, around 100 million years ago. Birds diversified dramatically around the time of the Cretaceous–Palaeogene extinction event 66 million years ago, which reduced the Pterosaurs, and killed off all the non-avian dinosaur lineages. Primitive bird-like dinosaurs have been found dating back to the mid-Jurassic period, around 170 million years ago. Many of these early "stem-birds", such as Archaeopteryx, were not yet capable of fully powered flight, and many retained primitive characteristics like toothy jaws in place of beaks, and long bony tails.